Rewrite common executables to use the Python selected by tox

[jaraco]

In jaraco/skeleton#86, @mtelka proposes to replace pytest in the tox commands with python -m pytest in order to avoid ambiguity in which Python version is used to execute the tests.

As described in that issue, making that change has other effects and would exist only to support a narrow set of uses.

Since tox-current-env is implicated in the failure and since it already re-writes the handling of python as an executable, perhaps this project should also handle other common executables (pytest to start, but maybe others) by replacing such invocations with python -m pytest or perhaps {sys.executable} $(which pytest).

This approach would address the issue more selectively and surgically where it's needed and avoid the need to adapt many (all?) downstream packages to use a non-standard approach to accommodate users of this project.

[hroncok]

Thanks for the report. This will be tricky.

Replacing pytest with python -m pytest has side effects.

Anyway, this limitation is documented: https://github.com/fedora-python/tox-current-env#do-not-rely-on-virtualenv-details